The script only does what you mentioned, viz., joins the removelist.txt items with the pipe character (alternation operator) for a !"or" regex of email addresses used by grep on the masterlist.txt lines.

To get a more efficient regular expression, use Regexp::Assemble to make your RE.

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James